Vapor-phase transport processes are of fundamental importance to the migration and persistence of volatile organic chemicals (VOCs) in subsurface environments. Such processes control the release of organic vapors into the atmosphere, dissolution of organic vapors into groundwater, and accumulation of fumes in basements, sewers, and underground utilities, which may pose the most immediate and widespread health risks following a VOC spill. Vapor-phase transport processes are also fundamental to the development and interpretation of soil-gas sampling methods and the design of effective soil venting remediation strategies. This research will focus on the following two aspects of vapor-phase transport which are not well understood; (a) multicomponent diffusion-dominated transport in natural soils and (b) the volatilization of entrapped organic liquids under forced- air advection. Emphasis will be placed upon the integration of laboratory and modeling studies. Subproject 0017 is designed to investigate the mechanisms governing vapor-phase diffusion in natural soils. The diffusive behavior of two model VOCs, benzene and trichloroethylene, will be explored. Experiments will be conducted to measure vapor-phase permeabilities, Knudsen diffusion coefficients, and tortuosity factors for a range of soil moisture, bulk density, and particle size conditions. In addition, the effect of vapor-phase sorption on organic vapor diffusion rates will be investigated for similar soils and experimental conditions. The utility of organo-soil complexes as a means of attenuating organic vapor mobility will also be evaluated. A multicomponent diffusion simulator, which incorporates several diffusion mechanisms and vapor-phase sorption, will be developed to assess the relative importance of these processes. Steady-state and transient column experiments will be performed to validate the modeling approach. In subproject 0006, experiments will be conducted to examine the rate of mass transfer of VOCs from the immiscible organic liquid phase to the vapor phase under steady-state and transient conditions. A number of organic compounds and soil matrices will be investigated. Correlations incorporating flow rate, composition of the organic phase, soil moisture content and soil structure will be developed for vapor-phase mass transfer. A multiphase flow and transport model, developed under current NIEHS funding, will be used to explore the implications of rate-limited mass transfer on the efficacy of soil venting technologies.